ubuntu@ubuntu:~$ sudo grub-install --root-directory=/mnt /dev/sda
/usr/sbin/grub-probe: error: cannot find a device for /mnt/.. (is /dev mounted?).
Fatal: Couldn't open either sysfs or procfs directories for accessing EFI variables.
Try 'modprobe efivars' as root.
Fatal: Couldn't open either sysfs or procfs directories for accessing EFI variables.
Try 'modprobe efivars' as root.
Installation finished. No error reported.